Various kinds of condenser microphones are available for various application purposes. The condenser microphones include a tie pin microphone attached to clothes or the like and a gooseneck microphone which is mainly used for conferences and disclosed in Patent Document 1 (Japanese Patent Application Publication No. H09-229292).
FIGS. 4 and 5 show conventional examples 1 and 2 of a tie pin microphone. FIG. 6 shows conventional example 3 of a gooseneck microphone. Of these drawings, FIGS. 4A and 5A are exploded views showing that an output module section is being assembled. FIGS. 4B and 5B are sectional views showing the completion of the assembly of the output module section.
In such tie pin and gooseneck condenser microphones, in order to make microphones inconspicuous, a condenser microphone unit 10 and an output module section 20 are separated from each other and are connected via a dedicated microphone cord 30.
Of the condenser microphone unit 10 and the output module section 20 which are shared among conventional examples 1 to 3, the condenser microphone unit 10 comprises a cartridge case 11 made of, e.g., aluminum. The cartridge case 11 acts as a shield case and houses a condenser unit including a diaphragm and a fixed pole (not shown) and an impedance converter. Generally, the impedance converter is an FET (field-effect transistor).
The output module section 20 comprises a cylindrical shield case 21 made of, e.g., a brass alloy. A circuit board 22 and an output connector 23 are housed in the shield case 21. A voice output circuit (not shown) including a transformer, a lowcut filter circuit, and an amplifier circuit, etc. is mounted on the circuit board 22.
The output connector 23 is fixed to one end of the circuit board 22 by screwing or the like and is disposed on the rear end of the shield case 21 when the circuit board 22 is housed in the shield case 21. The output module section 20 is called a power module section in some cases.
Generally, the output connector 23 in the condenser microphone is a 3-pin output connector defined by EIAJ RC5236 “a latch-lock round connector for sound.” To be specific, the output connector 23 comprises a first pin for grounding (shielding), a second pin used as the hot side of a signal, and a third pin used as the cold side of a signal. The output connector 23 is connected to a phantom power source via a balanced shield cable (not shown).
The microphone cord 30 is a twin-core shield covered wire (not specifically shown) which includes a power wire for supplying power to the condenser microphone unit 10, a signal line for transmitting a voice signal outputted from the impedance converter to the voice output circuit of the circuit board 22, and a shield covered wire for electrostatically shielding the power wire and the signal line and grounding the power wire and the signal line.
The shield covered wire of the microphone cord 30 is connected, on the side of the condenser microphone unit 10, to the cartridge case 11 and is connected, on the signal input side of the output module section 20, to a ground circuit (not shown) of the shield case 21 and the circuit board 22. The first grounding pin of the output connector 23 is connected, on the signal output side of the output module section 20, to the ground circuit of the shield case 21 and the circuit board 22 in a manner similar to the shield covered wire.
In conventional example 1 shown in FIG. 4, when the microphone cord 30 is connected to the circuit board 22 of the output module section 20, the microphone cord 30 is passed through the shield case 21 and soldered to the circuit board 22 while the circuit board 22 is drawn from the shield case 21 as shown in FIG. 4A. A knot 31 for preventing disconnection is tied at one end of the microphone cord 30.
Thereafter, as shown in FIG. 4B, the circuit board 22 is housed in the shield case 21 together with the output connector 23. In conventional example 1, a rubber bush 24 acting as a catch of the knot 31 is provided on the front end of the shield case 21.
In conventional example 2 shown in FIG. 5, a cord connector 40 is used to connect a microphone cord 30 and a circuit board 22 of an output module section 20. To be specific, one of connector members included in the cord connector 40, for example, a male connector member 41 is provided on the side of the microphone cord 30 and a female connector member 42 serving as the other connector member is mounted on the front end of a shield case 21. The female connector member 42 and the circuit board 22 are connected to each other via an internal cord 32.
As shown in FIG. 5A, in an actual connecting operation, one end of the internal cord 32 is connected to the female connector member 42, the female connector member 42 is attached to the front end of the shield case 21, and the other end of the internal cord 32 is drawn from the shield case 21 and soldered to the circuit board 22. Thereafter, as shown in FIG. 5B, the circuit board 22 is housed in the shield case 21 together with the output connector 23.
Conventional example 3 shown in FIG. 6 describes a gooseneck microphone where a condenser microphone unit 10 is supported by a flexible pipe 50. In this case, an end fitting 25 for fitting and fixing the flexible pipe 50 is provided on the front end of a shield case 21. A microphone cord 30 is routed into the flexible pipe 50 and drawn from the fixed end of the flexible pipe 50. A cord insertion hole 25a for inserting the microphone cord 30 into the shield case 21 is bored in the end fitting 25.
In conventional example 3, when the microphone cord 30 is connected to a circuit board 22 of an output module section 20, the fixed end of the flexible pipe 50 is, for example, press-fit and fixed into the end fitting 25 while the microphone cord 30 is inserted into the shield case 21 through the cord insertion hole 25a of the end fitting 25 as shown in FIG. 6A, and the microphone cord 30 is soldered to the circuit board 22 outside the shield case 21. Thereafter, as shown in FIG. 6B, the circuit board 22 is housed in the shield case 21 together with an output connector 23.
Incidentally, when strong electromagnetic waves are applied to the microphone cord 30, high-frequency current caused by the electromagnetic waves may enter the shield case 21, the high-frequency current may be detected by a voice output circuit formed on the circuit board 22, and noise may occur.
Cellular phones have rapidly become widespread in recent years. When a cellular phone is used near a microphone, extremely strong electromagnetic waves are received (for example, in a range of about several cm to several tens cm, an electric field is several tens of thousands times as strong as an electric field generated by commercial radio waves). Thus, the provision of solutions to cellular phones is an urgent necessity in the field of microphones.
Particularly when the microphone cord 30 has a long wire length in the output module section 20, a part having the long wire length acts as an antenna and thus is susceptible to influences of electromagnetic waves. Therefore, when considering measures against electromagnetic waves, it is more preferable that the microphone cord 30 have a shorter wire length in the output module section 20. However, in conventional example 1, the knot 31 for preventing disconnection has to be tied on the microphone cord 30, resulting in a long wire length in the output module section 20.
Further, in conventional examples 2 and 3, the internal cord 32 of conventional example 2 is included in the microphone cord 30, and the circuit board 22 has to be drawn out of the shield case 21 when the microphone cord 30 is soldered to the circuit board 22 and during maintenance (repair). Thus, the microphone cord 30 in the output module section 20 has to have a wire length equal to or longer than the shaft length of the shield case 21. Therefore, in the condenser microphones of conventional examples 1 to 3, particularly noise caused by electromagnetic waves is a serious problem.